Conventionally, an electronic power steering device for assisting a steering force of a driver with using a driving force of an electronic motor is well known. The steering device includes various fail-safe functions.
JP-A-2005-304119 teaches that the electronic motor includes multiple three-phase windings. An inverter for supplying current to the windings corresponds to the windings in each set. When a certain inverter or a certain windings in one set is broken, an open/close element stops supplying current from a broken inverter to corresponding windings. Thus, only a normal inverter supplies current to corresponding windings.
In JP-A-2005-304119, one set of the windings includes multiple windings in each phase. Multiple windings in one phase are arranged to have structurally the same direction of a phase difference in a circumferential direction. Thus, when a magnetic flux distribution provided by an arrangement of the windings is appropriately presented so that electro-magnetic attractive force generated between a stator and a rotor in a radial direction is cancelled, the electronic motor is stably operated even when only the normal inverter drives the motor.
Further, in JP-A-2009-6963, which corresponds to US 2010/0017063, the electronic motor includes one set of windings. One inverter supplies current to the three-phase windings. A relay is arranged between the inverter and the windings. The relay cuts the current to a certain phase winding independently.
When the inverter or the windings is broken, a broken phase winding or a broken phase element in the inverter is specified by a failure detector. The current flowing through the broken phase winding or element is cut by the relay. Then, only two phase windings or elements, which are normal, drives the motor.
In JP-A-2005-304119, when an inverter or three-phase windings in one set is broken, and only inverter and three-phase windings in another set, which are not broken, drive the motor, it is preferable to double the current to be supplied from the inverter to the windings. In this case, the magnetic flux generated in the stator is almost the same as normal operation. Accordingly, the driving force of the motor after failure is substantially equal to that before failure. However, when the driving force of the motor is maintained after failure, the driver does not feel strangeness of operation of the steering wheel. Thus, the driver is not aware of the failure of the power steering device.
Further, when the current to be supplied from the inverter to the three-phase windings is doubled after the failure, it is necessary to increase heat resistance of a semiconductor element and an electronic element, which are mounted on the motor. Accordingly, this means to increase a manufacturing cost of the semiconductor element and the electronic element although the cost is reduced because of multi-system of the driving circuit in the motor.
In JP-A-2005-304119, the relay for cutting current is arranged between each phase element of the inverter and a corresponding phase of the windings. When the relay is formed in the electronic power steering device for driving the motor with using a N-system driving circuit, the number of relays to be necessary is three multiplied by N. Accordingly, the number of parts in the power steering device increases, and the manufacturing cost increases.
Further, when the inverter or the windings is broken, the motor is driven by two-phase windings, which are normal. Thus, the electro-magnetic attractive force is inhomogeneous, so that the motor may vibrate. The vibration of the power steering device conducts the steering wheel, which is operated by the driver. Thus, the driver may feel anxiety.